Conventionally, technologies for controlling audio signal beams (i.e., sound waves converted into beams having directivities) by use of array speakers, in which a plurality of speaker units are regularly arranged so as to produce sounds, are known. For example, Japanese Unexamined Patent Application Publication No. H03-159500 and Japanese Unexamined Patent Application Publication No. S63-9300 disclose technologies regarding array speaker systems.
A control method for sound directivity in an array speaker will be described with reference to FIG. 7.
In FIG. 7, reference numerals sp-1 to sp-n designate speaker units that are linearly arranged with prescribed distances therebetween. In the case of generation of an audio signal beam emitted towards a focal point X, a circle Y whose radius matches a distance L from the focal point X is drawn. Delay times (=Li/speed of sound (340 m/s)) are calculated corresponding to distances Li between the speaker units sp-i (where i=1, . . . , n) and the intersecting points, at which the circle Y intersects line segments interconnecting the focal point X and the speaker units sp-1 to sp-n respectively, and the delay times are applied to input signals of the speaker units sp-i. Thus, it is possible to control the sound directivity of the array speaker in such a way that audio signal beams respectively emitted from the plural speaker units sp-1 to sp-n reach the focal point X at the same time.
As described above, prescribed delay times are applied to audio signal beams output from the speaker units so as to control the sound directivity of an array speaker in such a way that plural audio signal beams reach a prescribed point (or a focal point) desirably set in a three-dimensional space at the same time, whereby it is possible to obtain an effect as if prescribed sound was emitted in the direction towards the focal point.
According to an application of the aforementioned sound directivity control technology, a plurality of audio signal beams are reflected on a desired wall surface of a room so as to produce a virtual sound source thereon, whereby it is possible to realize a multi-channel surround effect.
FIG. 8 is an illustration showing an example of an application of the aforementioned sound directivity control technology, wherein reference numeral 81 designates a listening room; reference numeral 82 designates a video device such as a television set; reference numeral 83 designates an array speaker; and reference numeral 84 designates a listener. This performs 5.1 channel reproduction, wherein with respect to center channel (C) signals, an audio signal beam is emitted at a front side of the array speaker 83; with respect to main left channel (L) signals, an audio signal beam is controlled to strike a left-side wall surface of the listening room 81 so as to realize a virtual left channel 85; and with respect to main right channel (R) signals, an audio signal beam is controlled to strike a right-side wall surface of the listening room 81 so as to realize a virtual right channel 86. With respect to surround left channel (SL) signals, an audio signal beam is controlled such that it is reflected on the left-side wall surface and then strikes a rear-side wall surface, thus realizing a virtual surround left channel 87. With respect to surround right channel (SR) signals, an audio signal beam is controlled such that it is reflected on the right-side wall surface and then strikes the rear-side wall surface, thus realizing a virtual surround right channel 88.
As described above, by use of the array speaker 83, with respect to the L-channel signals, R-channel signals, SL-channel signals, and SR-channel signals, the corresponding audio signal beams are controlled to strike the prescribed wall surfaces of the listening room 81 so as to realize the virtual channels 85 to 88, whereby it is possible to perform three-dimensional sound control in such a way that the corresponding sounds can be heard by way of the virtual channels.
There also exist applied technologies in which different sound directivities are allocated to different contents so as to realize hearing of different contents in the left side and right side of a room respectively. This is disclosed in Japanese Unexamined Patent Application Publication No. H11-27604, for example.
As described above, it is possible to realize multi-channel reproduction and simultaneous reproduction of different contents by controlling audio signal beams in array speakers.
However, when audio signal beams are controlled in an array speaker, there exist problems due to differences of audio wavelengths. That is, in order to control signals of low-frequency ranges, it is necessary to adequately increase the overall width of an array speaker; but in order to control signals of high-frequency ranges, it is necessary to adequately decrease the distance between adjacent speaker units in the array speaker. For example, in order to control an audio signal beam by controlling side lobes of signals at the frequency of 10 kHz, which belongs to an essential audio frequency band, it may be ideal that the distance between adjacent speaker units be set to 3.4 cm (=speed of sound, 340 m/sec÷10 kHz), which matches the wavelength thereof or is lower. In this case, differences of delay times between adjacent speaker units are reduced to be very small.
The aforementioned phenomenon will be described in detail with reference to FIGS. 9A and 9B. These drawings show differences of delay times between adjacent speaker units (designated by reference symbols spa and spb) in an array speaker in which adjacent speaker units are each arrayed with a distance of 3.4 cm therebetween when an audio signal beam is controlled to be directed towards a focal point X, which is set 2 m distant from the front surface of the array speaker. In the case of FIG. 9A, the focal point X is set on the basis of a reference position that is 1 m distant from the speaker unit spb. In the case of FIG. 9B, the focal point X is set on the basis of a reference position corresponding to the position of the speaker unit spb.
Specifically, in the case of FIG. 9A, a distance of 2.2361 m lies between the speaker unit spb and the focal point X; and a distance of 2.2515 m lies between the speaker unit spa, which is adjacent to the speaker unit spb, and the focal point X, wherein a difference of delay times between the speaker units spb and spa is calculated as (2.2515 m−2.2316 m)÷340 m/sec=45 μs. When a delay time ta is applied to an input signal of the speaker unit spa, a delay time applied to an input signal of the speaker unit spb is represented as (ta+45 μm). In the case of FIG. 9B, a distance of 2 m lies between the speaker unit spb and the focal point X; and a distance of 2.0003 m lies between the speaker unit spa and the focal point X, wherein a difference of delay times between the speaker units spb and spa is calculated as 0.0003 m÷340 m/sec=0.9 μs. In this case, a delay time of (ta+0.9 μm) is applied to an input signal of the speaker unit spb.
As described above, a difference of delay times between adjacent speaker units may vary in response to the position of the focal point X; normally, however, it ranges from several tens of micro-seconds to one micro-second or less; that is, it is a very small time difference.
FIG. 10 shows a basic constitution of a delay control circuit (or an audio signal beam control circuit) for an array speaker, in which delay times are respectively applied to signals supplied to speaker units. This shows the circuitry that handles a one-channel signal, i.e., an audio signal beam only. The circuitry handling plural channels (or plural audio signal beams) can be realized by way of the addition for adding together delayed channel signals prior to D/A converters; hence, the circuit of FIG. 10 can be easily expanded.
In FIG. 10, reference numeral 91 designates an A/D converter; reference numeral 92 designates a delay memory having plural taps; reference numerals 93 designate multipliers arranged in connection with speaker units; reference numerals 94 designate D/A converters arranged in connection with speaker units; reference numerals 95 designate speaker units forming an array speaker; and reference numeral 96 designates a control means (i.e., a microcomputer) for setting up delay times, i.e., for making setup such that one of the taps of the delay memory 92 is to be connected to the multiplier 93 arranged in connection with a desired speaker unit 95.
In the delay control circuit having the aforementioned constitution, an analog input signal is converted into a digital signal in the A/D converter 91 and is then supplied to the delay memory 92. In contrast, a digital input signal is directly supplied to the delay memory 92 without the intervention of the A/D converter 91. The delay memory 92 is a shift register that is constituted by connecting together delay elements in plural stages in series, wherein the input signal thereof (i.e., the digital signal) is delayed by delay times, which are integer times greater than the sampling frequency, and is then output from each of the taps. The microcomputer 96 calculates a delay time to be applied to a desired speaker unit in response to the position of the focal point X, to which an audio signal beam is to be directed; then, the output of the tap of the delay memory 92 designated by the calculated delay time is selectively connected with a multiplier 93 in connection with the desired speaker unit. A delay signal output from the selected tap of the delay memory 92 is supplied to the multiplier 93 in which window processing required for audio signal beam control is executed and in which a volume gain is applied thereto; thereafter, it is converted into an analog signal in the D/A converter 94 and is then supplied to the corresponding speaker unit 95, thus realizing emission of a prescribed audio signal beam.
As described above, delay times to be applied to speaker units respectively are selectively set up in the delay memory 92, in which the taps are positioned such that a delay value corresponding to the sampling frequency forms a minimal unit of delay time.
FIG. 11 shows a detailed constitution of the delay memory 92, wherein reference numerals 92-1 to 92-5 . . . designate delay elements that are connected in series to form a shift register.
For example, when a delay time D1 is applied to an input signal of each speaker unit in synchronization with a sampling period T1, the number of taps for realizing prescribed delay times can be calculated by D1/T1.
The microcomputer 96 shown in FIG. 10 calculates distances with regard to speaker units distant from the focal point X; then, it calculates delay times applied to input signals of the speaker units, wherein the delay times are realized as delay-tap numbers with respect to the speaker units. The delay-tap numbers are calculated by rounding off any fractions from D1/T1. Suppose that the calculation result of D1/T1 is represented as (a+b) where “a” represents an integer part, and “b” represents a decimal part; and the shift register has an input X(z) and an output Y(z), wherein the following relationships are established.When b>0.5, Y(z)=X(z)z−a.When b≧0.5, Y(z)=X(z)z−(a÷1).
When the sampling frequency Fs is set to 200 kHz (i.e., sampling period T1=5 μs), and the applied delay time D1 is set to 17 μs, the calculation is performed as 17/5=3.4, wherein a=3 and b=4. In this case, b<0.5; hence, Y(z)=X(z) z−3.
This designates the extraction of a signal to which a delay time of 15 μs is applied by a tap of the delay element 92-3 within the plural delay elements forming the shift register of the delay memory 92, whereby an error of 2 μs occurs in comparison with a desired delay time of 17 μs.
As described above, when the sampling frequency Fs is set to 200 kHz, the minimum unit of delay time that can be set up becomes equal to 5 μs. This makes it difficult to realize desired differences of delay times between speaker units.
In order to increase the resolution regarding the delay time, it is necessary to increase the sampling frequency Fs; however, in order to realize delay times using small minimum units, a relatively large capacity of memory is required, and it is necessary to provide D/A converters and an A/D converter having high-speed processing capabilities. In addition, it is necessary to perform high-speed digital processing. This brings difficulty in circuit designing; and there occur problems due to increase of electric power consumption and high manufacturing cost. Furthermore, in the case of digital signal processing such as digital filtering, a further large number of taps (i.e., the number of operational circuits) must be required in order to realize prescribed characteristics. For this reason, numerous disadvantages may occur when the sampling frequency is increased in order to increase the resolution regarding the delay time.
This invention is made in consideration of the aforementioned circumstances; hence, it is an object of the invention to provide an array speaker system that can control directivities of audio signal beams, realized by array speakers, with high precision.